Capacitive microphones consist of two membranes: a membrane that is actuated by the sound pressure and a perforated membrane which forms a counter-electrode (“backplate”) that does not move in response to sound pressure, as the perforations render it acoustically transparent. The perforations allow the first membrane to move without pressure build-up in the volume between membrane and backplate.
FIG. 1 shows a top-view and cross-section through a (MEMS) condenser microphone.
The Si substrate 1 has an opening which exposes a part of the movable membrane 3 which is sensitive to acoustic pressure. The movable membrane is formed over an (optional) insulator 2. The backplate 5 (a fixed membrane) is suspended over a further insulator 4 and is perforated with a regular pattern of holes. The electrode connections 6,7 are to the two membranes and are used to measure the capacitance.
The intrinsic noise of a capacitance sensor can be made fairly small due to low mechanical losses and well-controlled processing and optimized design, so that the read out circuitry (typically CMOS circuitry) dominates the noise. A problem that remains is the small sensitivity of most MEMS devices under a constant-charge bias arrangement. This bias arrangement is typically used for MEMS microphones due to low power consumption and a stable operation point of the MEMS sensor.
The noise of the CMOS read-out circuitry becomes less important if the MEMS signal is strong. However, this is only possible at high voltages or close to the instable pull-in point of the sensor. The sensitivity is highest at this instable pull-in point. In fact, the sensitivity becomes infinite in the DC limit. The noise of the CMOS input stage then becomes less important.
A fast feedback control system can stabilize the movable part of the MEMS sensor at any point, including the instable point if the feedback bandwidth is considerably larger than the resonance frequency.
WO 2006/040403 discloses a feedback system for a MEMS sensor, which operates closes to the pull-in point. FIG. 2 shows the feedback circuit of WO2006/040403.
The circuit comprises a capacitive bridge 104,105,106,107 including the sensor capacitance 105. The capacitive bridge is regulated via a feedback loop and a bias resistor 103 such that it is at a constant operating point. With capacitor 107, the operating point can be set such that the membrane position is at the pull-in point. The feedback bias signal is the read-out signal. It is still non-linear, as the force-voltage relation is quadratic, but the signal is fairly linear for small signals.
The circuit has a regulator 101 which feeds an operational amplifier 102. The regulator 101 has a DC input supplied from a phase sensitive detector 101, which compares the bridge sensor signal (amplified by amplifiers 108,109) with a reference AC signal using a phase mixer 110.
The operation of the circuit is described in detail in WO 2006/040403.
In the ideal case, the gain of the sensor becomes infinite at the instable point. The total sensitivity is then only set by the feedback loop. The noise of the loop becomes insignificant because of the high gain of the low noise sensor.
Digital Σ-Δ (Sigma-Delta) feedback-loops are also known for MEMS sensors.
The known feedback loops however have some disadvantages:                a higher power consumption may result than for a DC readout.        the sensitivity (or sensor gain) is not infinite, even at the pull-in instable point since the mass acts as a dynamic spring constant. This leads to a frequency dependence of the transfer function.        the feedback loop may become unstable. In large microphone membranes for example, higher order modes exist. If they are excited by strong acoustic signals, then it is questionable if the feedback loop can suppress oscillation, because it is only designed for the fundamental deflection mode.        the optimum bias point can drift, e.g., due to temperature or ageing. It can also dynamically move with the feedback bias voltage as the electrostatic spring softening depends on the bias voltage. The feedback-loop of FIG. 2 only keeps the capacitance constant, i.e. the mechanical spring constant but not the bias voltage. The acoustic pressure can influence the mechanical properties. The electrostatic pressure and the acoustic pressure have different profiles so that effectively the membrane will deform even if the capacitance or the mean deflection is kept constant. The optimum point will therefore shift in response to a large input signal.        the process spread is not compensated. This requires tuning during testing or self-calibration.        
Thus, a need exists for better signal to noise ratios of MEMS capacitive sensors such as microphones, but also other pressure sensors, accelerometers, gyroscopes, etc. The intrinsic noise of a capacitance sensor can be made fairly small due to low mechanical losses and well-controlled processing and optimized design, so that the CMOS-read-out dominates the noise. MEMS capacitive sensors have small capacitances due to their small size. This means that the input stage of the read-out circuit needs a low capacitance. Unfortunately, the input noise of a CMOS read-out circuit increases when the input capacitance becomes smaller. New technologies do not improve this CMOS noise significantly. Options like bipolar JFET devices and cooling are more expensive in cost or power.